A short-arc high-pressure mercury lamp is widely used as a light source for a projection type image display apparatus (e.g., a liquid crystal projector). Such a short-arc high-pressure mercury lamp is a high-luminance light source that is close to a point light source and has a high color rendering index.
FIG. 13A is a cross-sectional front view showing the structure of an arc tube 141 constituting a conventional short-arc high-pressure mercury lamp. An enveloping vessel 141a of the arc tube 141 is made of fused quartz. The arc tube 141 includes a light emitting portion 142 and sealing portions 143 and 144. The light emitting portion 142 is in the center of the arc tube 141 and substantially spheroidal in shape. Each of the sealing portions 143 and 144 is substantially cylindrical in shape and connected to the light emitting portion 142 in such a manner that the sealing portions 143 and 144 extend from sides of the light emitting portion 142 outward in opposite directions.
Mercury (not illustrated) and the like are enclosed in the light emitting portion 142 as enclosed materials. Here, the mercury functions as light emitting material. The light emitting portion 142 further encloses portions of a pair of electrodes 145 and 146 made of tungsten, in such a manner that a first end of the electrode 145 and a first end of the electrode 146 are in opposition to each other. A discharge space 147 is also formed in the light emitting portion 142.
The electrodes 145 and 146 have electrode bars 145a and 146a, respectively. A cross section of each of the electrode bars 145a and 146a has a circular shape. Second ends of the electrode bars 145a and 146a are respectively joined, by welding, to strips of rectangular metallic foils 148 and 149 made of molybdenum.
Portions of the electrode bars 145a and 146a, including the second ends thereof, are embedded in and sealed in the sealing portions 143 and 144, respectively. Here, however, said portions of the electrode bars 145a and 146a being embedded in the sealing portions 143 and 144 does not mean that the entire circumferential surfaces of said portions of the electrode bars 145a and 146a are in contact with fused quartz in the sealing portions 143 and 144.
More specifically, some areas of the circumferential surfaces of the electrode bars 145a and 146a are in contact with the fused quartz; other areas inevitably do not come in contact with the fused quartz. In other words, small clearances exist around said other areas, and the enclosed materials in the light emitting portion 142 may enter said small clearances. In particular, as depicted in FIG. 13B which is an enlarged cross-sectional view of major components of the arc tube 141, a clearance X that is slightly larger than said small clearances is formed in an area where the electrode bar 145a (146a) lies on the surface of the metallic foil 148 (149).
In view of the above problem, thin metallic foils 148 and 149 having a thickness of 20 [μm] have generally been incorporate in the sealing portions 143 and 144, so as to leave no such clearances during the sealing process and to keep the sealing portions 143 and 144 airtight. Use of these thin metallic foils 148 and 149 alleviates the stress caused by the difference between (i) the coefficient of thermal expansion of the thin metallic foils 148 and 149 and (ii) the coefficient of thermal expansion of the fused quartz. This prevents small cracks from forming around the thin metallic foils 148 and 149.
In contrast, it is not possible, during the sealing process, to alleviate the stress caused by the difference between (i) the coefficient of thermal expansion of the electrode bars 145a and 146a and (ii) the coefficient of thermal expansion of the fused quartz. Therefore, small cracks form around the electrode bars 145a and 146a, as well as around joining areas where the electrode bars 145a and 146a are joined to the metallic foils 148 and 149.
In the above case, the only disadvantage is formation of small cracks. However, some high-pressure mercury lamps enclose a larger amount of mercury (e.g., 0.15 mg/mm3 or more) so as to increase vapor pressure during illumination and achieve higher luminance. With such high-pressure mercury lamps, there have been cases where small cracks, which originally formed on a minuscule scale, grew in size under the stress caused by a high vapor pressure during illumination; this would eventually lead to damage to the sealing portions 143 and 144.
One conventional method to suppress such damage to the sealing portions 143 and 144 (more specifically, to reduce the clearance X in size or remove the clearance X) is to (i) reduce the width of a first end of a metallic foil to which a second end of the electrode bar is to be joined, (ii) wrap the first end of the metallic foil around a portion of the circumferential surface of the electrode bar, and (iii) join the second end of the electrode bar to the first end of the metallic foil by welding (for example, see Patent Document 1).
Another conventional method to suppress such damage is to attach a cylindrical member made of metal, or a metallic coil, to a second end of the electrode bar that is to be joined to a metallic foil in the sealing portion, so that the cylindrical member or the metallic coil functions as a cushion between the electrode bar and the fused quartz. Use of this method alleviates the stress caused by the stated difference between the coefficients of thermal expansion (for example, see Patent Documents 2 and 3). Here, the width of the first end of the metallic foil to which the second end of the electrode bar is to be joined has been reduced. The cylindrical member or the metallic coil covers the first end of the metallic foil, together with the end of the electrode bar which is joined to the first end of the metallic foil.
Patent Document 1:                Japanese Patent Application Publication No. 2003-257373        
Patent Document 2:                Japanese Patent Application Publication No. 2001-189149        
Patent Document 3:                Japanese Patent Application Publication No. 2003-187747        